The dye-sensitized solar cell is called a wet solar cell, a Graetzel cell, or the like, and has an electrochemical cell structure as typified by an iodine solution without using a silicon semiconductor. Specifically, the dye-sensitized solar cell has a simple structure in which an iodine solution or the like is disposed as an electrolyte solution between a porous semiconductor layer (porous oxide semiconductor layer), such as a titania layer, formed by baking titanium dioxide powder or the like onto a transparent conductive glass plate (a transparent substrate having a transparent conductive film laminated thereon and serving as an anode substrate) and making a dye adsorbed onto the baked powder, and a counter electrode composed of a conductive glass plate (conductive substrate serving as an cathode substrate). Electrons are generated as the result of solar light introduced into the dye-sensitized solar cell from the transparent conductive glass plate side being absorbed by the dye.
The dye-sensitized solar cell is inexpensive in terms of the materials thereof and does not need a large-scaled facility for its fabrication, thus attracting attention as a low-cost solar cell.
While the dye-sensitized solar cell is currently reported to have a solar light conversion efficiency of 11%, further improvements are required to be made to the conversion efficiency. Thus, studies have been made from various viewpoints.
As one of the studies, attempts have been made in various ways to improve optical absorption efficiency.
That is, various researches have been heretofore made on dyes Used in the dye-sensitized solar cell. However, any dyes capable of absorbing light having a wide wavelength range from 400 nm up to a near-infrared or longer wavelength with high efficiency have not been obtained so far. Consequently, light not absorbed by a porous semiconductor layer having a dye adsorbed thereto directly results in absorption loss. Note that it is conceivable to increase the thickness of the porous semiconductor layer having a dye adsorbed thereto, for the purpose of increasing optical absorption efficiency. In practice, however, this may not lead to absorption efficiency improvements for various reasons, but contrarily may cause absorption efficiency to decrease.
In order to improve optical absorption efficiency, a study is being made of a so-called tandem-type dye-sensitized solar cell in which two porous oxide semiconductor layers each carrying a dye are provided in series with respect to the traveling direction of light.
For example, a study has been made of a technique to extract electricity in parallel from two cells configured by laminating over each other two porous semiconductor layers having dissimilar dyes adsorbed thereto and arranging an electrode containing an FTO (fluorine-doped tin oxide film) between the two porous semiconductor layers (see Non Patent Literature 1).
In addition, a study has been made of a dye-sensitized solar cell in which a first anode having a first sensitizing dye and a second anode having a second sensitizing dye different from the first sensitizing dye are disposed separately from each other, a cathode typically made of a mesh electrode is provided between the two anodes, and an electrolyte is filled between these respective electrodes (see Patent Literature 1).
The patent literature states that incident light input from the first anode side can be efficiently utilized by parallel-connecting two battery cells formed on both sides of the cathode of this cell.
Still additionally, a study has been made of a dye-sensitized solar cell in which anodes provided with porous titanium oxide films each having a sensitizing dye adsorbed thereto and a cathode serving as a counter electrode are disposed separately from each other, a transparent counter electrode is provided between each anode electrode and the cathode electrode, and an electrolyte is filled between these respective electrodes (see Patent Literature 1). In this case, the anode-side surface of an insulating layer formed as an intermediate layer of the transparent counter electrode functions as the cathode electrode and the cathode-side surface of the insulating layer of the transparent counter electrode functions as the anode electrode. Consequently, the dye-sensitized solar cell is similar in configuration to the dye-sensitized solar cell of Patent Literature 1 described above in which the two battery cells formed on both sides of the transparent counter electrode are parallel-connected (see Non Patent Literature 2). Note that this Non Patent Literature 2 does not disclose any more information than is deciphered from drawings, and therefore, the materials, structure and the like of the transparent electrode are uncertain.
The inventors et al. of the present invention have proposed a dye-sensitized solar cell, though not such a tandem-type dye-sensitized solar cell as described above, in which two porous semiconductor layers are arranged, a conductive layer (collecting electrode) having a through-hole between the two porous semiconductor layers is provided, and the conductive layer is electrically connected to a transparent conductive film of a transparent substrate provided on an optical incidence side (see Patent Literature 2).
According to this dye-sensitized solar cell, high conversion efficiency can be obtained even if the thickness of the porous semiconductor layers is increased.
A study has been made of another dye-sensitized solar cell, specifically an np tandem-type dye-sensitized solar cell, in which an anode substrate, a dye-sensitized n-type semiconductor layer, an electrolyte layer, a dye-sensitized p-type semiconductor layer, and a cathode substrate are disposed in this order (see Patent Literature 3).
The patent literature states that, according to this dye-sensitized solar cell, the conversion efficiency of the cell as a whole can be improved by improving p-side conversion efficiency by reducing p-side electrical resistance.